Microwave wattmeter



H. E. WEBBER MICROWAVE WATTMETER 5 Sheets-Sheet 1 Filed. Dec. 6. 1943 QN NQ lNvEmoR HUGH ,Wt-3BE l ATTORNEY condition of constant current supplied thereto.

Patented June ,14, 1949 Hugh E.

Webber, Williston Park, N. Y.,

assigner to The Sperry Corporation, a corporation of Delay Waffe Application December .6, 1943, Serial No. 513,164

My and concerns, particularly, methods 'and apparatus for the measurement of power at high ire'- quencies such as in the microwave region'. An object of the invention isl to providea novel alternating current power wattmeter employing a concentric line structure having a portion of its inner conductor serving as a hot wire element and using a stub line for supplying direct current to the hot wire element. A further object of the invention is to provide a Bridge compensated for variation in ambient temperature of the hot wire element.

An object is to provide a concentric line hot wire holder which may readily be opened for replacement of the hot wire unit.

Another object is to which may readily be adjusted or tuned for measuring high frequency power at different frequencies.

An additional object is to provide a power measuring bridge suitable for measurement of large amounts of power.

Further, an object of vide a broad band wattmeter and a wire holder therefor ohviating the necessity for tuning.

Other and further objects and advantages will become apparent as the description proceeds.

A method of measuring alternating current power has been proposed in which a hot wire element or Wollaston wire is fed with both a1- ternating current and direct current, and variation in the magnitude of the alternating current power is detected by observing the variation in the .direct current power required to maintain.

the total power dissipation ofthe hot wire constant. Since the resistance of the wire depends upon the temperature and the'temperaturein turn depends upon total power dissipation may be maintained rather maintaining it `constant by varying the direct Since direct current `instruments may be produced with a high degree of accuracy compared with the accuracy of alterhating current instruments,- particularly highv accurately by measuring the direct current resistance of the wire and invention relates to power lmeasurement hot-wire constant it may provide a hot wire holder the invention is to pro- 13 Claims.- (Cl. 171--95l For readily maintaining be made one arm of a direct-current bridge or a bridge supplied with low-frequency alternating-current equivalent to direct-current in comparison with radio-frequency current.

In order that microwave energy may be fed to the wire, and a direct current connection may also be made thereto without interfering with the microwave circuit, a suitable hot wire holder of the concentric line type, for example, is provided. The wire is mounted within the outer cylinder of'a concentric line, and the wire itself forms a part of the inner rod or conductor. In order to avoid reflection of microwave energy or high standing wave ratios, the line is either tuned to eliminate reflections from the hot wire or the hot l wire is mounted at the current anti-node or loop and a shorting stub is provided for bringing in a direct current connection without interposing any alternating current impedance.

, When the broad band eiect is desired, that is, when it is desired to measure power accurately over a range of frequencies without readjustment of the apparatus. the hot wire is preferably mounted asv close as possible to a point in the 'concentric line which is a dead short with respect to high frequency current.

For high power measurements the hot wire is mounted in a bulb iilled with hydrogen, and for very low power measurements the wire is drawn to a very ne diameter and prepared by special methods to avoid rupture of such a fragile wire "during the process of mounting. For a given microwave band width both'length and diameter of the hot wire is limited to make certain that dil rect-current and radio-frequency resistances are equal. l.

the power dissipation, the 40?.,

v2.5176143717 V,and to iiled February 4,

- Thisfapplication isrelated to' my Patents Nos. 2,419,613;- 2,424,596; 2,414,499; 2,468,793 and `application Serial No. 429,508, l i942, in the names of William W.v Hansen, John R. Woodyard and Edward L. GlnZtOn.

A better understanding of the invention will i be afiordedby the following detailed description frequency instruments., this substitution method -5 enables accurate measurements of alternatingv considereci` in conjunction with the accompanying drawings, in which Fig. 1 is a view partially in section of a microwa vewattrr'ieter having a wire holder of the con- 3 a centric line type represented as partially cut by a plane passing through 'the axes of both the main line and the lateral shorting stub;

Fig. 2 is a longitudinal section oi the apparatus of Fig. 1, represented as cut by a plane 2 2 through the axis of the main line and perpendicular to the plane dened by the axis of the main line and the shorting stub in Fig. 1;

Fig. 3 is a circuit diagram of a power measur ing bridge in which a hot wire mounted in a concentric line may be employed as one arm of the bridge;

Fig. 4 is a view, mainly a longitudinal section. of a wire holder for use in a relatively narrow range of frequencies;

Fig. 5 is a cross-sectional view of a portion of the apparatus of Fig. 4, cut by a plane 5 5 as seen when looking in the direction of the arrows:

Fig. 6 is a corresponding cross-sectional view of a portion of the apparatus of Fig. 4, cut by a plane 6 6;

Fig. 'l is a cross-sectional view of a hot wire holder in which the hot wire element is mounted at a current anti-node;

Fig. 8 is an enlarged detail view of the hot wire unit shown in Fig. 7.

Like reference characters are utilized throughout the drawings to designate like parts.

Fig. 1 illustrates a wire holder and a hot wire suitable for use in a bridge circuit, such as shown in Fig. 3. One arm II of the Ibridge represents the hot Wire holder of Fig. 1. The bridge shown in Fig. 3 comprises in addition to the hot wire unit II, a compensating resistor I2, a standard bridge arm I3 and a fourth bridge arm |4, consisting of a resistor I5, and a current responsive instrument such as a milliammeter I6 in series. A conventional null-indicatlng galvanometer I1 and a sensitivity-adjusting rheostat I8 are connected in series between conjugate points |9 and 20 of the bridge to form a conventional diagonal bridge arm. A source of direct current 22 in series with rheostats 23 is connected to the remaining conjugate points 24 and 25 of the bridge. As shown, a switch 26 is connected in series with the power current source 22.

The resistances of the bridge arms are so chosen that at a predetermined temperature of the hot wire in unit II, the bridge will be in balance and this temperature is usually the one existing when the maximum safe power is being supplied to the hot wire unit II. For maximum sensitivity, the resistances of the four arms may be made substantially equal. The resistance of the resistor I5 is so chosen that the sum of its resistance and the resistance of the milliammeter I6 will equal the desired resistance for the bridge arm I4.

When the resistances of the four arms are made equal the total direct current power inputto the bridge may readily be calculated from the measured current input and the resistance of the arms. The direct power consumption in unit is then one-fourth the power input. However, by making the resistances of the arms of II and I4 equal regardless of the resistances chosen for the arms I2 and I3, and connecting the milliammeter I6 in series with the resistor I5 to form the arm I4, direct current power consumption in the arm I| may be calculated directly from the reading of the instrument I6 and the resistance of the The wire holder shown in Fig. 1 comprises a main concentric line 21 consisting of an outer cylinder and an inner rod and a stub line 28 also y the line 21,

rod. Both lines consisting of an outer conductor and an inner 21 and 28 are made adjustable in eirective electrical length for tuning purposes. The outer conductor of the main line 21 comprises tubular members 29 and 3|. The members 29 and 3| are arranged to be separable but are designed to be fastened together in a xed relation during operation of the apparatus by.a threaded sleeve coupling 33.

The inner conductor of the main line 21 comprises a hollow rod 39 secured at the lefthand end of the tubular member 29 by an insulating bushing .4|, a hollow rod 42, threaded to a solid rod 43 extending to the righthand end of the tubular member 3| and supported at the righthand end by an insulating bushing 44, and a barretter wire or a Wollaston wire 46 mounted in an evacuated tube 45 to form the unit II with wire terminals 41 and 48 making contact with the tubular rods 39 and 42, respectively. y

'I'he bulb 45 of the hot Wire element l I is shown with a tip 49 at the side resulting from the evacuation and sealing oil of the bulb. To provide space for the tip 49, the connector sleeve`33 is therefore madev of suillcient diameter and an opening 5I is left at one side of the tubular member 29 to receive the tip 49.

For varying the electrically ellective length of a slidable line closer is provided comprising wipers 52 adapted to make electrical connection with the inner surface of the tubular member 3|, carried by a hollow member 53 which is slidably supported by the rod 43 through insulating bushings 54 and 55. For further supporting the member 53, the rod 43 is provided with a sleeve 56 secured thereto, and for insulating an enlarged portion from the sleeve 56 a cylinder of insulating material 58 is interposed between the members 56 and 51.

For setting the member sition it is secured to a sleeve 34 which is in turn adapted to be adjustably secured to the tubular member 3| by means of a screw 36 reaching through a slot 35 in the tubular member 3| with a, head 31 inside the tubular member 3| and a securing nut 38 adapted 53 in longitudinal powavelength adjusting kto bear against the outer surface of the wavewire 46 from another concentric transmission' line (not shown) a conventional concentric line coupling 64 vis provided at the lefthand end of the line 21.l It coupling provides for making electrical connection to the outer conductor 29 and the inner conductor 39, from corresponding elements of the supply line (not shown).

For making direct current connections with the barretter wire 46, a, coupling unit 65 is provided comprising an inner terminal 66 making electrical contact with the internal rod 43 of the concentric line 21, and an outer screw thread terminal 61 making electrical connection with the outer conductor of the tubular member 3| -through a metallic member 68. As previously 51 of the hollow rod 53.

will be understood that such a mentioned the insulators Il. 54 and Il are provided to prevent creating a direct-current short circuit between the tubular member Il and the inner rod d3. In order that a direct-current return path from the lefthand terminal I1 of the v t6 may be made through the outer barretter wire conductor of the concentric line without-shortcircuiting or producing reflections in the high frequency circuit, the stub line 28 is provided..

The stub line 2B comprises an outer conductor 6i, which is electrically connected to the outer conductor members it and ti of the main line il, and an inner conductor rod 'i Il, which is electrically connected to the inner conductor member i@ of the main line il. For tuning out any refiections which might tend to be causedby the presence of the wire it in the inner conductor oi the line il, and for making the electrical length of the stub line it such as to interpose no impedance in the main line il, the stub line it is so constructed that it may be varied in wavelength in a manner similar to that described. ior the main line il. Thus, the line it is also provided with spring contact members 'it carried by a member 'it and a longitudinal adjusting sleeve 'it provided with coarse and micrometer adjustments as in the case of the sleeve tt.

it wili be understood that when the electrical wavelength of the line it is equal to an odd number oi `quarterwaves oi. the microwave energy ied to the line 2i, a direct connection between the inner and outer conductors ci the upper end of the stub line it will have no edect on the main line il. ln order to avoid redections irom the righthand end ci the line il back toward the input end ti the righthand end oi the electrical portion of the line il should constitute a short circuit with respect to radio frequency. This is accomplished by making the conducting members iii and bt and the insulating sleeve iid between them of suiicient length to form a iby--pass condenser. Therefore, with respect to high irequency. the members di and bt may be considered to be electrically connected.

it direct-current circuit may be traced from the direct-current terminal tt through the rod it, the barretter wire terminal it, the wire itt, the wire terminal conductor ii, the member iii, the contact fingers it, the outer conductor tit-oi the stub line tt, the cuter tubular member it oi the main line il, the outer tubular member ti thereof, the metallic ntting tt and back to the second direct-current terminal ti.

For the purpose of compensating the bridge of Fig. 3 for variations in ambient temperature, which would tend to vary the resistance ci the wire tir' for a predetermined power dissipation, a compensating resistor may be mounted in close proximity to the wire it so as to be subjected to substantially the same ambient temperature. This resistor may be one oi the arms of the bridge, for example, the arm ii as previously mentioned.

For mounting the resistor i2 in close proximiity to the wire it, a resistor spool may be formed around the barretter wire and bulb it, comprising retaining discs or flanges 'it formed in a sleeve it and in the coupling sleeve it, see Figs. l and 2. Resistance wire of suitable size is wound between the flanges 'i5 to produce a total resistance of the value desired for the arm it of the bridge. Since the wire oi the compensating resistor is not protected by. enclosure in an evacuated bulb, it will be understood that the wire will need to be till, the inner conductor tu, inner justment of relatively heavy and therefore made o! considerable lensth in comparison with the length and thickness ofthe wire 48.

When a power measurement is to be made, be fore the microwave energy is supplied to the coupling 64, the bridge of Fig. 3 is balanced by adthe rheostats 23 until the current owing through the barretter wire 46 is such as to produce a. standard resistance of the bridge and causing null indication of the galvanometer I1. The milliammeter iii is then read im ascertain the direct-current power consumption oi the arms ii or it. Thereupon the direct-current power supply to the bridge is reduced to prevent overloading and burning out of the wire it, and microwave power is supplied to the wire it through the connection mi. The rheostats it are again adjusted until a balance is obtained by the null indication oi the galvanometer il. The milliammeter it is again read and the value of the alternating power input is then determined by the dierence between the direct power inputs to the wire it before and after admission of the alternating current power.

When relatively little frequency variation in the microwave'input is to be expected, the longitudinal adjustment of the main line 2i is not needed and the stub line it is utilized only for making a direct current return connection irorn the lefthand terminal il oi the barretter wire dii. For example, as shown inv Fig. fi, the wire holder may take the form of a concentric line unit hav-fl ing a main line il and stub line iti. The main line 211 comprises an outer cylinder "ii, the lett hand inner rod :it and the barreter wire unit il;

the stub line it comprises an outer cylinder 'iii and an inner rod lid. A longitudinally slidable, adjustable-ratio impedance-metching transformer or matching sleeve tt may be provided.

For the purpose of shorting the righthand end oi the main line 2l with respect to alternatingr current a by-pass condenser ti. is formed at the righthand end of the main line il. The by-pass condenser tl comprises a metallic fitting di with a tapered or conical outer surface and a second fitting t3 with a tapered inner surface mating the outer surface of the fitting iii, but separated therefrom by insulating sheet material 3d to form a condenser. The tting ci is electrically connected to the terminal it oi' the rwire it by a suitable means such as by means of an abutting screw tt, and the fitting dit has a hollow portion tb, adapted to receive a conventional direct current coupling t5', and a neck portion tl ttin inside and making electrical contact with the righthand end of the outer conductor member lll. The direct current coupling tt' includes an outer threaded terminal ed', making electrical contact with the member d3, and an inner terminal titi' connected by a conductor te to the fitting di.

For adjusting electrical length of the stub line 2t the inner surface of the cylinder 'it is threaded and a threaded nut or plug di is provided which carries contact iingers di, engaging the inner conductor rod i9. A closing cap 92 may be provided. It will be understood that the plug 39 is provided with recesses (not shown) whereby a suitable tool may be utilized for rotating the plug 39 in the threaded inner surface of the cylinder i8.

In this case the direct-current electrical circuit from the inner connector teminal 66' takes place through the conductor 88, the fitting t2, the barretter wire terminal 48, the barretter wire M. its terminal 41, the inner conductor 39, the inner l conductor 19, contacts 9|, plug 89, cylinder 18 and the outer cylinder 11 back to the neck portion 61 of the fitting 33 and the threaded terminal 61'.

To permit mounting and replacing the barretter wire unit I, the fitting 33 is made removable, the inner conductor 39 is made hollow at the end and it is provided with a set screw 93 for receiving and fastening the terminal 41 of the Wollaston wire 46. An opening (not shown) is provided in the cylinder 11 to provide access to the screw 93 by means of a screwdriver. To provide space for the top 49 of the bulb 45, a channel 94 (Fig. 5) is provided in the side of the hollow cylinder 11, and the sides of the slots 94 are provided with longitudinal ribs 95. For securing the fitting 83 to the righthand end of the hollow cylinder 11 a clamp (Fig. 6) is provided comprising a split ring 96 secured to clamping jaws 91 cooperating with a clamping screw 98.

When broad band characteristics are required for the wattmeter, a wire holder may be employed in which the barretter wire is mounted at the current anti-node or loop in a concentric transmission line. As illustrated in Fig. 7 the wire holder comprises a concentric transmission line IOI and a lateral stub |02 with a conventional input connection I 03 at the lefthand end of the line for receiving power from a concentric line (not shown), and a direct lcurrent connector |04 at the righthand end.

The main line IOI comprises a hollow-cylindrical outer conductor |05, an inner conductor rod |06 and a barretter wire unit |01. The lateral stub |02 comprises a hollow-cylindrical outer conductor |08 joined tothe cylinder |05 and a center rod |09. For electrically insulating the righthand end of the hot wire unit |01 from the outer conductor |05, with respect to direct current while maintaining a short circuit at the righthand end of the line with respect to alternating current, a by-pass condenser is formed at the righthand end of the line |0|. Such a condenser may comprise a pair of concentric cylinders ||2 and II3 with mating tapered adjacent surfaces separated by a sheet of insulating material |I4. The outer member |3 is in electrical contact and joined to the inner surface of the outer cylinder |05; and the inner member I|2 is provided with an internal flange H6 for making electrical contact with the righthand end of the 'barretter wire unit |01.

The barretter wire unit |01, which will bedescribed in greater detail hereinafter in connection with Fig. 8, includes a lefthand terminal I|1 fitting into a suitable axial opening in the rod |06 and a righthand terminal or metallic end plate I |8 adapted to fit against the ange |I6 of the condenser member |I2. The direct current connector |04 includes an inner prong terminal ||9 andan outer threaded terminal |2I. For securing the outer threaded terminal I2| in the righthand end of the hollow cylinder |05 and making electrical connection therebetween a metallic bushing |22 is provided. A pierced insulating plug |23 is provided for supporting the prong I|9 within the outer threaded terminal |2|.

For holding the plate I 8 of the wire |01 against the iiange II6 in the by-pass condenser I I and for making an electrical connection between the connector prong |I9 and the end plate I|8, a spring unit |24 is provided comprising a tube |25 provided with internal flange |26 at the llefthand end, secured at the righthand end to the insulator plug 23 and containing acompression spring I 21, and a spring-pressed contact pin |28 protruding 8 the opening at the lefthand end of the tube and having a head |29 abutting the spring |21 within the tube |25.

For terminating the stub line |02, a wave trap I3I is provided, comprising a pair of concentric hollow cylinders |32 and |33, internally and externally spaced from, but electrically and mechanically connected to a hollow cylindrical extension |34 of a hollow longitudinally slidable plug |35 f1tting the inner surface of the hollow cylinder |08. As shown, the hollow cylinders |32 and |33 are electrically connected to the hollow cylinder |34 at the lower end thereof by means of an annular member |36. The hollow cylinders |32 and |33 are spaced from the inner conductor rod |09 and the outer conductor hollow cylinder |08 as well as from the hollow cylinder extension |34, and are spaced also from shoulders |31 in the plug |35. 'I'he cylinders |32 and |33 are made substantially one-quarter wave in length for the average wavelength of microwave frequency to be l measured or one-half wavelength from the radio frequency end (over one-quarter wavelength section and down to bottom or dead short again) in order to provide a lcomplete termination at the lower end of the plug |35.

To permit adjustment of the electrical length of the stub line |02 in terms of wavelengths, the hollow cylinder |08 may be provided with slots |38, and screws |39 cooperating with the slots |38 and threaded into plug |35 may be provided.

is entirely symmetrical with respect to the longitudinal axis through its terminal rod I|1, has no laterally extending sealing tip and utilizes a fiat end plate terminal I|8. This shape and structure facilitates the mounting arrangements. permits the simpliiicationflof the construction of the outer cylinder |05 and also makes it possible to connect the righthand end of the power-absorbing wire of the unit |01 directly at the current anti-node.

As shown in |01 lugs |44 and |45 welded to the ends of the wire |43 and in turn welded or soldered to the tube |42 and they end plate |I8, respectively.

' The glass of which the tube I 42 is composed, and the metal of which the end plate I I8 and the tube |42 are composed are preferably so chosen as to have the same temperature coefiicient of expansion. For example, relatively low temperature coeicient of expansion material may be employed such as an alloy known as Kovar alloy, for the elements ||8 and |42, and a glass such as that known as 9X or 705 glass may be employed for the tube I4|.

As many changes could be made in the above construction and many apparentlyvwidely different embodiments of this invention could be made Without departing from the scope thereof,

d it is intended that all matter contained in the above description or shown, in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is: l l l. A hot wire holder for a high frequency wattmeter, comprising in combination, a concentric transmission line having an outer hollow cylindrical conductor, an inner conductor with a gap therein, a barretter wire unit to complete the inner conductor, a stub line comprising an outer hollow cylindrical conductor laterally extending from and electrically connected to the outer conductor oi. the inst-mentioned line, an inner conductor electrically connected to the inner conductor of the mst-mentioned line, means for closing one end of the line with respect to high frequency currents, but insulating the inner and i outer conductors with respect to direct current, and means for supplying high frequency energy to the other end of the line.

2. Apparatus as in claim l, with means for adjusting the electrical lengths of the main line and the stub line.

3. Apparatus as in claim 1 in which the outerv hollow cylindrical conductor of the main line is separable andra coupling sleeve is provided for removably joining the separable portions to provide for installation and replacement of the barretter wire uni-t. f

Il. Apparatus as in claim l having a spool formed around the portion oi the outer cylindricalv conductor surrounding the barretter wire unit for receiving ambient temperature compensating resistance wire. l

5. An alternating current power-measuring bridge for high frequency measurements, comprising in combination, a transmission line, a pair of resistors connected in series to form two bridge arms, a current-responsive instrument, a third resistor connected in series with said current-responsive instrument to form a third bridge arm, and a resistor within said transmission line, the resistance of which depends upon measurement conditions forming a fourth bridge arm, means for supplying electrical energy to said bridge for maintaining the resistance of said fourth arm substantially constant, the nrst two resistors having a fixed ratio while simultaneously the third and fourth bridge arms have the same ratio for a predetermined measurement condition whereby the power consumption in the current-responsive instrument and the resistance of the third bridge arm when a balanced condition is maintained.

6. A hot wire unit holder ior a microwave wattmeter, comprising a transmission line of the concentric type, and a stub line, the concentric line comprising an outer conductor in the form of a hollow cylinder, an inner conductor extending coaxialy within said cylinder for a portion oi the length thereof, a ed within the said cylinder forming a continuation of said inner conductor and a by-pass condenser having an inner member electrically connected to said hot wire u 't, an outer member electrically connected to the outer cylinder, and an insulating sheet between said two units, said insulating unit cylinder least comparable with the diameter `of the cylinder whereby a low-impedance radio-,frequency path is provided at the end of the Atransmission line between the inner and outer conductors,

hot wire element mountbut y tion for ythe line rod and from the outer conductor as atrasos said conductors are insulated with respect to direct current.,

7. A wire holder for a broad band microwave wattmeter of a ban'etter wire type, comprising a transmission line having inner and outer conductors, a by-pass condenser at one end of the line and said inner conductor extending only part way toward the end of the line to providev space for interposing a barretter wire unit, and a tuning stub extending laterally from the transmission line at the end of the inner conductor of the transmission line.

8. Apparatus as in claim 'l wherein the tuning stub is terminated by a wave trap.

9. An alternating current power-measuring system comprising a transmission line having inner and outer conductors, a by-pass condenser at one end of the line, a power-absorbing resistance wire mounted within said line adjacent said by-pass condenser and constituting a portion of the inner conductor thereof, and a, tuning stub extending laterally from said transmission line, said tuning stub comprising a hollow outer conductor, an inner conductor rod substantially centrally disposed therein and extending throughout the length of said outer conductor, and a hollow plug adjustabl-y slidable in said outer` conductor and on said inner conductor for making sliding electrical contact therewith, said plug having a hollow cylindrical extension surrounding said rod, a pair of concentric hollow cylinders internally and externally spaced from said hollow cylindrical thereof, said cylinders being also spaced from the well as from said plug, whereby a non-contacting terminais formed at the end ci said hollow cylindrical plug extension.

10. Apparatus as in claim 9, wherein the hollow cylinders are substantially a quarter-wave in length for the average frequency for which they transmission line is designed. i

11. A high-frequency wattmeter having a conl centric transmission line comprising an outer and fourth bridge arm may be calculated from the current flow in the having actual length at. v

and a tubular outer conductor, a resistance eleductor adjacent inner conductor, a barretter wire unit interposed therein constituting a portion of said inner conductor and a tuning stub line extending laterally from said transmission line having an outer hollow conductor, and an inner conductor rod extending throughout said outer hollow conductor. said termination comprising a hollow supporting member surrounding said rod and extending toward one end thereof, a pair of concentric hollow cylinders internally and externally spaced from, but electrically and mechanically connected to the hollow supporting member at one of their ends, and so dimensioned as to be spaced also from the center rod and outer conductor, the concentric hollow cylinders being insulated from said supporting member at their ends opposite from the end at which they are supported and means for moving said supporting member and said cylinders for varying the electrical length of said transmisson line.

. l2. Apparatus as in claim 1l, wherein the concentric hollow cylinders are substantially onequarter wave in length for the average frequency for which the coaxial line is designed.

13. In a microwave measuring instrument, the combination of a section of coaxial transmission line-short-circulted at one end for high frequency waves, said line comprising an inner conductor ment connected in series with said inner conextension and carried by the end to said end, adjustable sus ceptance means vconnected across said line at the 4 end of said resistance element remote from said one end, said line having a nxed length adjacent said element, said susceptance means cooperating with said fixed length to match the conductance of said element with the characteristic conductance of said line at the end oi' said nxed length remote from said element, nxed susceptance means on said inner conductor at said last end.

HUGH E. WEBBER.

` REFERENCES CITED The following ref are of record in the `fue of this patent:

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12 Name Date Runaldue May 18, 1937 Dow Mar. 1, 1938 Browder et al May 21, 1940 Meulendyke Mar. 4, 1941 Brown Nov. 11, 1941 Dow May 26, 1942 Mason May 26, 1942 Alford et al. Sept. 8, 1942 Brown Mar. 23, 1943 Tawney Apr. 6, 1943 Dow et al Apr. 10, 1945 George Apr. 30, 1946 Harrison May 6, 1946 Oki-ess May 21, 1946 Clifford et al. Dec. 24, 1946 FOREIGN PATENTS Country Date Great Britain Nov. 17, 1937 Germany Apr. 12, 1938 

